Some 27 species originate from Central America, but are scarcely ever seen in cultivation. The vast majority of dahlias grown for their ornamental flowers are of hybrid origin, usually of obscure parentage. Many thousands of such cultivars have been described, of which a few hundred are available commercially. D. rosea Cav. (syn. D. variabilis Desf.) and D. juarezii hort. (the cactus dahlia) are the chief parent species, with D. pinnata Cav. and D. coccinea Cav. also being involved in the evolution of the modern cultivars. The genus is classified in the tribe Heliantheae.

A gardener's boy, aged 15 years, who had worked for three years in a dahlia nursery, developed dermatitis of the face and hands. Patch tests with leaves and flowers were negative, but there was a strong reaction to the tuber (Vryman 1933). A recurrent summer eruption in a woman was traced to a dahlia; the leaf was found to produce a positive patch test reaction in the patient, negative in controls (Calnan 1973a). Calnan (1978b) also described the case of a female (55 year old) market gardener who had recurrent dermatitis of the hands, extensor surfaces of the arms, and around the mouth and eyelids. Patch tests to dahlia flower gave a 2+ response; chrysanthemum and primula (fam. Primulaceae) both gave 1+ responses. Cueva & Mingramm (1955) mentioned that three species of Dahlia in Mexico were capable of causing dermatitis.

Cross-sensitivity to several members of the Compositae including a plant described as Dahlia variabilis was found in patients with contact allergy to liverworts of the genus Frullania Raddi, fam. Jubulaceae (Fernandez de Corres & Corrales Torres 1978). It is probable that the authors were referring to an otherwise unidentified dahlia cultivar.

Frain-Bell & Johnson (1979) observed positive patch test reactions to the oleoresin from an unidentified Dahlia in 5 from 55 patients with the photosensitivity dermatitis and actinic reticuloid syndrome.

Dahlia pinnata Cav.

McCord (1962) noted the commercial availability of an oleoresin extract of this species for patch testing. However, there appear to be no reports of dermatitis from this plant.

Dicoma niccolifera Wild

This species, which grows almost exclusively over nickel-rich substrates in a limited area of Rhodesia (Zimbabwe), is capable of hyperaccumulating nickel. A concentration of 2100 ppm in the dried plant material has been reported (Brooks 1980). The contact sensitising properties of nickel and its salts are well documented (Malten et al. 1976, Cronin 1980).

This is a recently described genus of 2 species previously classified in the genera Erigeron L. and Inula L.

Dittrichia graveolens Greuter

(syns Inula graveolens Desf., Erigeron graveolens L.)

Stinkwort

The barbed pappuses can cause mechanical injury. Dermatitis occurred in workmen who pulled up this weed, only some of those exposed being affected (Maiden 1918a). The plant has been reported to produce dermatitis venenata in Australia (Cleland 1925, 1931). MacPherson (1932) reported dermatitis in a father and two sons. Other reports (Helms 1897, Wettenhall 1925, Seddon & Carne 1928, Hurst 1942, Scott 1967) suggest irritancy to man and animals.

Burry et al. (1973) reported four cases of contact sensitivity to the plant in 13 patients with "Australian bush dermatitis". Positive patch test reactions to the leaf and cross-sensitivity to other members of the Compositae were observed. They found that the dried leaf was not irritant by patch test. Later, Burry (1979) reported a positive patch test reaction to stinkwort (Inula graveolens) in a patient who had "fleabane dermatitis" (see Conyza bonariensis Cronq.); and Burry (1980a) observed a weakly positive patch test reaction to stinkwort in a 75 year old female patient who had been admitted to hospital with a severe exacerbation of a condition that had been diagnosed four years previously as photodermatitis.

Burry & Kloot (1982) consider that airborne dusts formed by the breakdown of senescent tissues of this and other species of Compositae are the causative agents of Australian bush dermatitis. These authors also give a distribution map for this species in Australia.

A fluorescent volatile oil is obtained from the plant (Watt & Breyer-Brandwijk 1962).

A potentially allergenic sesquiterpene lactone has been reported from this species.